1. Field of the Invention
The present invention relates to an exhaust system means for automobiles and a method of producing the exhaust system means. In more detail, the present invention relates to a corrosion-resisting exhaust system means for automobiles and a method of producing such an exhaust system means.
2. Description of the Prior Art
In coping with an increase of the exhaust gas temperature caused by the strict regulations with regard to the exhaust gas purification in recent years, the exhaust system means for automobiles such as an exhaust pipe or a muffler is now more frequently made of a steel material having an anti-corrosive surface layer such as an aluminum plating or zinc plating in order to ensure a long life. By employing these materials the durability of the exhaust system means is substantially improved when compared with the case of employing a normal steel material.
However, an exhaust system for automobiles generally comprises a lot of joining portions such as a joining portion of an exhaust pipe and a muffler, a joining portion of a pipe end and a flange, a joining portion of an exhaust pipe or a muffler and a supporting member which mounts the exhaust pipe or the muffler to the body of an automobile, and since these joining portions are generally joined by welding, there is a problem that the welded portions and their adjacent portions are relatively soon corroded. This is due to the fact that the welding bead itself is rather poor in corrosion resistance and that the anti-corrosive surface layer located adjacently around the welded portions is broken, cracked or deteroriated by the heat generated in the welding process. The problem of corrosion at the welded portions and the surrounding portions is particularly serious in cold districts such as the North American and European districts, because in these districts rock salt is sprinkled on the road in the winter season for preventing the freezing of the road surface, the salt becoming attached to the welded and surrounding portions while the automobile is running and acting as a corrosion accelerator for these portions.
In order to avoid local corrosion in the welded and surrounding portions, it has been contemplated to join the members in the exhaust system by clamping or caulking instead of welding or to apply a metal powder painting or an anti-corrosive painting to the welded and surrounding portions. However, a joint formed by clamping or caulking in an exhaust system will cause a problem with regard to gas tightness and, furthermore, such a joint is inferior in mechanical strength. The anti-corrosive painting is a problem with regard to its reliability and durability because the paint film can be easily removed when a small mechanical force has been applied thereto. It has also been contemplated to produce the entire exhaust system of a highly corrosion-resisting material such as a stainless steel and to weld them together by employing a similar highly corrosion-resisting welding material. However, this idea is not practical for the automobile industry because of its high cost.